fix layer ID key in get_existing_layer()
fix layer ID key in get_existing_layer()

get_existing_layer() failed because the key "id" cannot be found (tested
with WWT version 5.2.9). The correct key should be "ID", as returned by
LCAPI layerlist.

vga101/pywwt,jzuhone/pywwt,jzuhone/pywwt,vga101/pywwt